This invention relates in general to tube launched projectiles and in particular to tube launched projectiles which are fin stabilized or spin stabilized, where means are provided to elongate the projectile body when the projectile exits the launch tube.
Reduced length projectiles allow for reduced cost of transport and for increased launch tube propellant charge. In the case of fin stabilized projectiles the center of pressure of the fins can be relatively close to the projectile center of gravity. This configuration requires the fins be larger than for an elongated projectile to provide a restoring moment for control of the flight of the projectile. Since projectile fins typically contribute 30% to 50% of the total projectile aerodynamic drag, reductions in projectile drag would be desirable.
The present invention overcomes the disadvantages of fixed length projectiles detailed above by mounting the empennage or tail section on a hollow shaft which surrounds a portion of the projectile boom extending from the projectile body. The shaft is caused to slide along the boom in a constrained manner. The shaft is provided with axially extending splines spaced around its circumference which are assembled into matching grooves extending along the length of the boom. Thus, the projectile is assembled initially with the shaft in the most forward position. Means are provided for retaining the shaft in the forward position until the projectile exits the launch tube. The shaft is caused to slide to a rear, in-flight position after exiting the launch tube and is locked in the flight position during flight. The mechanism for extending the projectile to its flight length configuration utilizes the high pressure gas from the burning propellant to initially lock the shaft and boom in the pre-flight position during transit in the tube and to cause the shaft to slide to the flight position after exiting the launch tube. As an alternate or additional embodiment, a separate solid propellant or compressed gas cylinder may be provided in the projectile boom to cause the shaft to slide to the flight position.
A 10% to 25% reduction of projectile aerodynamic drag can be realized by the present invention whereby the projectile body elongates when the projectile has cleared the launching tube. This elongation occurs by causing the empennage containing the fin structure to slide rearward to a new flight position, effectively moving the fins rearward and achieving the advantages discussed above. Similarly, the rearward movement of the tail section and the accompanying shape change allows for significant changes in the stability characteristics and reduction of aerodynamic drag of spin-stabilized projectiles.
This invention is applicable forfin-and spin-stabilized tube launched projectiles. The empennage comprised of fins (fin-stabilized) or the aft body without fins (spin-stabilized) is mounted on a hollow shaft and at atmospheric ambient pressure is free to slide on a matching boom attached to the aft end of the projectile. At atmospheric ambient pressure, a small clearance between shaft and boom, say 0.001 inch, allows the shaft to move freely relative to the boom. Rotation between the shaft and boom is prevented by the following construction: axially extending slots or grooves provided in the boom, along with axially extending splines provided in the forward end of the shaft allow for the axial sliding of the shaft relative to the boom without rotation.
In my aforementioned patent 1 describe the shaft as deflecting onto the boom such that the two surfaces mechanically xe2x80x9clockxe2x80x9d onto one another at the high tube pressures accompanying propellant ignition, whereupon the shaft moves with the boom inside the tube. I describe this xe2x80x9clockingxe2x80x9d as being attained on deflection by providing each surface with intermeshing ridges running orthogonal to the projectile axis, and/or by matching engaging teeth, and/or the like, and/or by providing a high friction coefficient between the two surfaces. Thus, at the high tube pressures, the mechanical xe2x80x9clockxe2x80x9d is comprised of either of one or of a combination of intermeshing surfaces and friction shear stresses between the inner shaft and outer boom surfaces. Hence, following ignition of the projectile propellant, except possibly for an initial small insignificant movement until the tube pressure becomes sufficiently large, the shaft is xe2x80x9clockedxe2x80x9d onto and moves with the boom during the projectile transit in the tube.
I have now discovered an alternative means for locking the shaft onto the boom by using the axial or longitudinal force resulting from the differential pressure between the external and internal surfaces of the shaft vertical end wall (i.e., the aft end wall) created by the elevated tube pressure. This differential pressure acting on the shaft vertical end wall causes the shaft to travel with the boom during the shaft""s transit in the tube. The xe2x80x9clockingxe2x80x9d and conjoined travel ceases upon exposure of the shaft to the reduced ambient atmospheric pressure as the projectile egresses from the tube. In order for this type of xe2x80x9clockingxe2x80x9d to occur, the ratio of the axial acceleration of the shaft to the axial acceleration of the main body projectile must be equal to or greater than one. Stated mathematically:
aS/ap=(pS/pp)(AS/Ap)(mp/mS)xe2x89xa71,
where: xe2x80x9caxe2x80x9d represents axial acceleration; xe2x80x9cpxe2x80x9d represents differential pressure; xe2x80x9cAxe2x80x9d represents area normal to the launch tube centerline; xe2x80x9cmxe2x80x9d represents mass; and the subscripts xe2x80x9cSxe2x80x9d and xe2x80x9cPxe2x80x9d represent the shaft and main body projectile, respectively. AS does not include the area of the orifice opening 22 and any other open area not developing the differential pressure pS. This formula applies for materials of any friction coefficient and dictates the accelerations required to xe2x80x9clockxe2x80x9d the boom and shaft during transit in the launch tube. Inherent in the formula is the launch tube pressure (p) developed after the propellant charge is ignited in the launch tube, and which is approximately equal to ps and pp.
This method of axial xe2x80x9clockingxe2x80x9d effected by the differential pressure on the shaft vertical end wall can be utilized by itself or in conjunction with the mechanical locking described in my aforementioned patent.
An orifice at the base of the shaft serves as the opening to a small cavity within the boom. Propellant gases enter the cavity and the cavity pressure rises as a consequence of the high tube pressures during the transit of the projectile in the tube. Upon the projectile exiting the tube and the accompanying reduction of ambient pressure, the differential pressure across the shaft vertical end wall drops so that the force on the end wall of the shaft is removed, causing the shaft to xe2x80x9cunlockxe2x80x9d and again be free to move relative to the boom. Furthermore, the high pressure of the gas trapped in the cavity relative to that of the ambient atmosphere acts on the vertical end wall of the shaft producing a rearward movement of the shaft relative to the boom and thereby a lengthening of the projectile. The shaft slides along the boom and the projectile continues to lengthen until the splines in the shaft reach a taper lock in the boom, where it becomes jammed and locked into place on the boom. The time required for the shaft deployment depends in part on the cavity volume, orifice diameter, and deployment length. The shaft deployment and thereby the projectile length can be increased by several calibers within a short distance of the tube exit.